A(1). Field of the Invention
The invention relates to an arrangement for generating a clock signal from a received data signal located in a base band, comprising a zero-crossing detector having connected thereto a frequency selective device from which the clock signal is derived.
A(2). Description of the Prior Art
A generally known method of transmitting binary data symbols without the use of direct current and having sufficient clock information is bi-phase modulation, the bit rate being the carrier frequency. In this method there is a signal transition comprising the clock information in the centre of each symbol interval (FIG. 2). Other signal transitions which are determined by the data, occur between the symbol intervals. The signal transitions can be detected by a zero-crossing detector and are then applied to a phase-locked loop which is synchronized at twice the bit rate. A signal having the bit rate is derived therefrom by a divide-by-two frequency divider.
In another known method of clock generation the data signal is pre-processed in a non-linear manner and thereafter a signal having the bit rate is derived from the pre-processed data signal by means of filtering. This method, which is often used when the received data signal itself does not contain frequency components which are coupled to the bit rate requires an averaging of the phase fluctuations over a longer period of time.
In some methods of coding data symbols to obtain an amplitude-spectrum of particular shape, disturbing signal transitions occur which do not furnish reliable clock information. These unwanted signal transitions can be masked by keying pulses when they occur at instants which are clearly distinct from the desired signal transitions, which in practice, is not always the case, particularly in the case of transmission over a cable.